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Comment by adrian_b | Hacker News Reader

There were only 2 possible causes of the incident, either the bolts had been replaced with bolts made from an inappropriate material, so they broke, or the bolts had not been assembled correctly, and they became loose.

It appears that the latter is what happened.

Perhaps those who did the assembly of the doors at Boeing did not use the right kind of washers that are needed to prevent the unscrewing of the bolts, or they did not apply the correct torque to the bolts.

It is extremely surprising if such trivial errors can happen during the assembly of an aircraft.

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> There were only 2 possible causes of the incident, either the bolts had been replaced with bolts made from an inappropriate material, so they broke, or the bolts had not been assembled correctly, and they became loose.

Or the ‘correct’ assembly instructions weren’t actually correct, or the design isn’t good enough (e.g. it uses too few bolts, or didn’t check the strength of the bolts when they get cold), or the bolts were of the correct material, but designed too thin, etc.

There's at least 1 million root causes of the incident and while I have my own predictions, this industry is a great example of checking all possibilities before jumping to "2 possible causes".

It's really unlikely that washers are a root cause (spring/split washers are useless and are definitely not used for this type of apllication). "Correct torque" is not trivial to determine and maybe it was calculated right or wrong based on right or wrong materials/conditions or maybe conditions were changed or maybe...

I am 99.9% sure a procedure was followed and a torque or other measurement applied and was documented. This is probably not a trivial error but a cascading failure in design and/or documention and/or integration.

The torque or fastener spec could just be wrong in the face of unforeseen resonance or some other in-flight oddity cumulatively loosening the bolts.

I wouldn't be so certain it's installation error or mfr defect of the fasteners...

You should contact Boeing with that information, I'm sure they'll find it invaluable.

You are assuming this is problem in the assembly and not in the design?

The same part was used on the 737-900 for quite a while and didn't (as far as we've heard, at least) have any similar issues there.

And you seem to be assuming that the problem is in the design and not the assembly...

The latter seems more likely.

You are both missing the 'faulty process' and 'faulty communications' options.

And:

- "bad parts from vendor" - "bad programming on torque wrench from MRP" - "insufficient training on process" - "tooling was programmed correctly but bad sensors" - "lube mislabeled for 3 days"

This is absolutely not some simple thing. This is why people don't take us seriously as engineers.

For loose bolts, incorrect assembly is by far the most likely cause.

It can be a design problem if the bolts and associated parts like washers have been substituted recently or if the assembly instructions have been changed recently, e.g. by specifying a different torque. If any such engineering change has happened recently, then that would be the likely culprit.

A resonance problem of the aircraft body as supposed by another poster seems extremely unlikely as that would have required significant recent changes to the aircraft body, which did not happen.

Or the lock wire was left off or done incorrectly

Aren’t there multiple signoffs & inspections to see if the lock wire was being done correctly?

No matter the real root cause, surely multiple layers of precautions must have failed to allow this to happen, right? So given that's a foregone conclusion, how can we discredit a hypothesis based on that?

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